AVS 65th International Symposium & Exhibition
    Thin Films Division Tuesday Sessions
       Session TF-TuM

Paper TF-TuM13
ALD of Cobalt Phosphate Electro-catalyst for Oxygen Evolution Reaction

Tuesday, October 23, 2018, 12:00 pm, Room 101A

Session: Emerging Applications for ALD
Presenter: Valerio Di Palma, Eindhoven University of Technology, The Netherlands
Authors: V. Di Palma, Eindhoven University of Technology, The Netherlands
G. Zafeiropoulos, DIFFER
M.N. Tsampas, DIFFER
W.M.M. Kessels, Eindhoven University of Technology, The Netherlands
M.C. Creatore, Eindhoven University of Technology, The Netherlands
Correspondent: Click to Email
Sunlight-driven water splitting is widely investigated through studies addressing both the development of earth-abundant element-based (photo)electrodes, as well as engineering of compact, efficient and inexpensive devices. One important reason that keeps these systems from being of practical use to date is the sluggish kinetics (high overpotential) of the oxygen evolution reaction (OER). OER is a four electron-proton coupled reaction, while hydrogen evolution reaction (HER) is only a two electron-transfer reaction, and hence usually OER requires a higher overpotential [1].

Electro-deposited cobalt phosphate (CoPi) has been reported as a valid OER catalyst (as bare electrode or as cocatalyst in photoelectrodes) and alternative to rare element compounds [2]. In parallel, ALD is rapidly reaching out also to (photo-)electrocatalytic applications [3,4] and in the present contribution we investigate CoPi growth by ALD and its application as OER catalyst. Compared to electrodeposition, ALD offers the major advantage of tuning the electro-catalyst chemical composition, as we demonstrate in this work.

The CoPi process has been developed using an ABCD scheme, where A, B, C and D are the cobaltocene (CoCp), first oxygen plasma (5 s), trimethylphosphate (TMP) and second oxygen plasma (2 s) dosing steps, respectively. The deposition performed at 300oC results in a growth per cycle of about 1.1 Å, determined by in-situ spectroscopic ellipsometry. Elemental characterization by XPS of the ALD prepared CoPi shows a stoichiometry of Co3(PO4.3)2. The film deposited on FTO/glass is active for OER, as confirmed by cyclic voltammetry. The current density peak measured is about 1.0 mA/cm2 at 1.4 V vs. NHE (Normal Hydrogen Electrode), comparable to the values reported in literature for electrodeposited CoPi. Furthermore, we investigated the effect of Co-to-P concentration ratio on the OER catalytic activity. To this purpose, the process was modified from the aforementioned ABCD approach into an (AB)x(CD)y supercycle approach. The sample prepared with a ratio x-to-y of 12-to-11 showed already a higher current density peak (about 1.2 mA/cm2 at 1.4 V vs NHE) than the sample deposited without the use of supercycles. This finding highlights the role of Co-to-P concentration ratio on the activity of the catalyst and how the ALD supercycle approach can be adopted to this purpose.

References

[1] J. Surendranath et al., J. Am. Chem. Soc., 132 (39) (2010) 13692

[2] M.W. Kanan and D.G. Nocera, Science, 321 (2008) 1072

[3] A. Paracchino et al., Energy Environ. Sci., 5 (2012) 8673

[4] S.C. Riha et al., ACS Nano, 7 (3) (2013) 2396